1. Field of the Invention
This invention relates to a phase shift mask. More particularly, it relates to an attenuating phase shift mask which attenuates light of exposure wavelength and is suitable for a KrF excimer laser and especially an ArF excimer laser and an F2 excimer laser, a blank therefor, and a process of fabricating the phase shift mask blank.
2. Description of the Related Art
A high resolution and a depth of focus (DOF), which are significant characteristics required in photolithography, conflict with each other. It has been revealed that a resolution cannot be improved simply by increasing the numerical aperture of an optical aligner and shortening the exposure wavelength (see Semiconductor World, vol. 12 (1990) or Ohyo Buturi, vol. 60, No. 11(1991)).
Under these circumstances, phase shift lithography has been attracting attention as a photolithographic technique of next generation and been partly introduced. Phase shift lithography is a technique for improving the resolution in photolithography by alteration to the mask with no alterations to the optical system, in which a phase shift mask is used to change the phase of the light passing through the shifter of the mask so that the light having passed through the shifter and the light not having passed through the shifter are made to interfere with each other thereby to markedly increase the resolution.
A phase shift mask is a mask having light intensity information and phase information and includes a Levenson type, an auxiliary pattern type, and a self-alignment (edge enhancement) type. Compared with ordinary photomasks having only light intensity information, the phase shift masks are structurally complicated and need high technology to produce.
In recent years, half-tone phase shift masks, another type of phase shift mask, have been developed. A half-tone phase shift mask has translucent areas which have a function of substantially blocking light of exposure wavelength combined with a function of shifting (usually reversing) the phase of light. Since there is no need to separately form a light blocking pattern and a phase shifting pattern, the half-tone phase shift mask has a simpler structure and is easier to make.
Patterning of a blank for making a half-tone phase shift mask is achieved by dry etching. Where the light blocking function and the phase shifting function are performed by the respective layers, highly strict control is required for satisfactory patterning for each of the layers. On the other hand, the half-tone phase shift masks, in which single-layer translucent areas perform both of these functions, can be obtained through a single dry etching step. Thus, half-tone phase shift masks can be produced through a simplified process with a satisfactory pattern.
As shown in FIG. 1, a half-tone phase shift mask comprises a transparent substrate 100 having formed there on a masking pattern composed of light-transmitting areas (areas where the transparent substrate 100 is exposed) 200 which allow light of intensity contributory to exposure to pass through substantially and translucent areas (light-blocking and phase shifting areas) 300 which allow light of intensity not contributory to exposure to pass through substantially (FIG. 1A). The translucent areas 300 are designed such that the phase of the light transmitted therethrough is shifted to be substantially reversed with respect to that of the light transmitted through the light-transmitting areas (FIG. 1B). Light rays incident upon the vicinities of the border between a translucent area and a light-transmitting area are diffracted to invade and cancel out mutually. As a result, the light intensity in the border is reduced to practically zero thereby improving the contrast, namely, the resolution (FIG. 1C).
The translucent areas of a half-tone phase shift mask or a translucent film of a half-tone phase shift mask blank before patterning (the term xe2x80x9ctranslucent filmxe2x80x9d will hereinafter be used to describe both the translucent film of a blank and the translucent areas of a mask) should have a suitable transmittance and cause a suitable phase shift as a phase shifter. Specifically, the translucent film should have (1) a controlled transmittance within a range of from 3 to 20% for an exposure wavelength of a KrF excimer laser, an ArF excimer laser, etc., (2) capability of shifting the phase of the light of the exposure wavelength usually by 180xc2x0, and (3) a sufficient transmittance (usually 65% or higher) for wavelengths used for mask inspection, e.g., 257 nm, 266 nm, 364 nm, and 488 nm.
Further, the translucent film should have sufficient resistance against acids (e.g., sulfuric acid) and alkali solutions (e.g., ammonia) because the blanks or masks are to be cleaned with an acid solution in a patterning process or on use.
It has been taught that molybdenum silicide oxynitride (MoSiON) is capable of forming a single-layered shifter showing the above-mentioned characteristics (see JP-A-6-214792 and Japanese Patent Nos. 2878143 and 2989156).
In order to secure a DOF in exposure, the translucent film of the half-tone phase shift mask or mask blank is also required not to have such a large internal stress as to deform the transparent substrate, on which the translucent film is formed. A sufficiently small internal stress is particularly demanded for use with an ArF excimer laser (wavelength: 193 nm).
However, as the wavelength used for exposure has become shorter and shorter (from i-rays (365 nm) to KrF laser rays (248 nm) and from KrF laser rays to ArF excimer laser rays (193 nm)), the above-described half-tone phase shift masks of related art and processes of fabricating them have turned out to involve the following problems.
After the light transmittance and the phase shift of the translucent film comprising molybdenum silicide oxynitride are once set to fit for an ArF excimer laser, they are liable to deviate from the set values when the blank or the mask is pretreated or cleaned in the fabrication process or on use because the conventional molybdenum silicide oxynitride film has insufficient resistance against an acid solution (e.g., sulfuric acid) and an alkali solution (e.g., ammonia).
Deviation of the phase shift depends on the change in thickness of the translucent film caused by cleaning during mask fabrication. Deviation of a phase angle is represented by formula (1):
[360(nxe2x88x921)d]/xcexxe2x80x83xe2x80x83(1)
wherein n is a refractive index of a translucent area at an exposure wavelength; d is a thickness change of the translucent area caused by cleaning with an acid or alkali solution; and xcex is an exposure wavelength.
As is understood from formula (1), the shorter the exposure wavelength, the greater the phase angle deviation, with the thickness change being fixed. Accordingly, a phase shift mask should have an increased resistance to acids and alkali solutions with shortening of the exposure wavelength. That is, phase shift masks and their blanks for an ArF excimer laser are particularly required to have improved acid and alkali resistance for practical use.
Another problem is that laser light gains in energy with wavelength shortening to damage the translucent areas. It follows that the phase shift mask suffers from deviations in transmittance and phase shift from initially set values within the expected service life. Therefore, the phase shift mask for an ArF excimer laser is additionally required to have particularly improved durability against irradiation with an excimer laser.
As long as a KrF excimer laser is used as a light source, the state-of-the-art half-tone phase shift masks and blanks are practicable and yet desired to have improved resistance to acid solutions, alkali solutions, and irradiation with an excimer laser.
Accordingly, an object of the present invention is to provide a half-tone phase shift mask blank and a half-tone phase shift mask of which the translucent film has improved acid resistance, alkali resistance and resistance to excimer laser irradiation to cope with shortening of an exposure wavelength.
Another object of the present invention is to provide a half-tone phase shift mask blank and a half-tone phase shift mask of which the translucent film has improved acid resistance, alkali resistance and resistance to excimer laser irradiation over the state-of-the-art ones.
Conceivable approaches to improve acid resistance, alkali resistance and resistance to excimer laser irradiation of a phase shift mask include an alteration to the composition of the translucent film and an increase of the translucent film""s density.
Seeing that an alteration to the translucent film""s composition is greatly influential on the transmittance and the phase angle, a lot of labor will be involved for composition designing while fulfilling all the characteristics required of a phase shift mask blank. A process for obtaining an appropriate composition is disclosed in Japanese Patent 2989156 supra.
To increase the density of the translucent film, it is effective to reduce the pressure of the nitrogen-containing atmosphere in which the translucent film is formed by sputtering. Compared with the alteration to the composition, this approach is advantageous in that the influences on the transmittance and phase angle of the film can be minimized. In silicon nitride (SiN) film formation by sputtering, however, a reduction in pressure of the sputtering atmosphere results in an increase of internal stress of the film (J. Electrochem. Soc., vol. 137, No. 5, pp. 1582-1587 (May, 1990)). The same problem arises in forming films having a high silicon nitride content, such as a molybdenum silicide nitride film or a molybdenum silicide oxynitride film as contemplated in the present invention.
Accordingly, still another object of the present invention is to provide a half-tone phase shift mask blank and a half-tone phase shift mask of which the translucent film has improved acid resistance, alkali resistance and resistance to excimer laser irradiation while maintaining the internal stress of the film within an acceptable range for the intended use.
A first aspect of the present invention is a half-tone phase shift mask blank which comprises a transparent substrate having provided thereon a translucent film comprising at least one thin layer containing silicon and at least one of nitrogen and oxygen and which is to be exposed to light whose center wavelength is 248 nm or shorter, wherein the translucent film is dense such that it has a center-line surface roughness (Ra) of 0.3 nm or smaller.
A second aspect of the present invention is a half-tone phase shift mask blank which comprises a transparent substrate having provided thereon a translucent film comprising at least one thin film containing silicon and at least one of nitrogen and oxygen and which is to be exposed to light whose center wavelength is 193 nm or shorter, wherein the translucent film is dense such that it has a center-line surface roughness (Ra) of 0.2 nm or smaller.
Another aspect of the present invention is a half-tone phase shift mask blank which comprises a transparent substrate having provided thereon a translucent film containing a metal, silicon, and at least one of nitrogen and oxygen and which is to be exposed to light whose center wavelength is 248 nm or shorter, wherein the translucent film is a film formed by sputtering a target containing the metal and silicon in an atmosphere containing at least one of nitrogen and oxygen and having a pressure of 0.2 Pa or lower and then heat-treating the formed film at a temperature of 200xc2x0 C. or higher.
Yet another aspect of the present invention is a process of fabricating a photomask blank comprising a transparent substrate having provided thereon a single-layered or multi-layered thin film for patterning, which comprises forming at least one layer of the thin film by sputtering in an atmosphere having a pressure of 0.15 Pa or less.